This invention relates to reprocessing for recycling of scrap plastic, reprocessing solid thermoplastic waste materials resulting from unsuccessful plastic molding operation and to reprocessing of solid plastic mold runners and sprues produced in the ordinary course of plastic injection or compression molding.
The plastics industry uses granulators to recycle discarded solid plastic waste and to recycle scrap materials which are produced incident to the production of injection molded or compression molded plastic products and for recycling defective molded products, where molding has been unsuccessful due to incomplete mold fill or unsatisfactory conditions within the mold.
Many plastic resin materials used in compression and injection molding may be repeatedly melted and formed. Thus, if a molded part is formed incorrectly or the desired shade of color is not obtained, the part can be ground into small particles and processed again.
In addition to scrap and malformed parts, parts are sometimes molded attached to a runner, which is a solid plastic tree connecting together two or more parts produced in separate cavities in the same mold. These runners may also be recycled; manufacturers utilize granulators to do this.
Sprues are solid plastic material resulting from cooling of molten plastic material in one or more passageways between the mold cavities and the point of introduction of molten plastic material into the molding machine.
Granulators are normally selected based on size(s) of plastic parts the granulator must ingest and required maximum throughput of such parts in pounds per hour. In a plastic molding facility there is often one granulator adjacent to every molding press, with the granulator dedicated to grinding and returning to the process defective parts molded on that press as well as runners and sprues resulting in the molding process.
Known granulators employ a reel-type blade assembly similar to that of a reel-type, hand-operated, manually powered lawn mower. Such conventional granulators typically have two heavy side plates with bearings. Connecting these plates are heavy cross bars holding bed knives, which are generally two (2) in number, positioned on opposite sides of the reel axis and oriented parallel to the axis of rotation of the reel. Journaled in the bearings retained by the side plates is a heavy reel which holds three (3) and sometimes five (5) reel knives. Plastic material to be granulated for recycling enters the cutting region perpendicular to the axis of reel rotation.
In known granulators, distance between the side plates may typically be about twelve (12) inches and cutting circle diameter may also typically be about twelve (12) inches. Hence the knives are about twelve (12) inches long. The possibility of having to make a single twelve (12) inch long cut, along the full length of a blade, for one hit on a single chunk of plastic determines required power capacity for the motor, which is typically 5 or 10 horsepower even if gear reduction is used. Flywheels are often utilized to aid the motor in the cutting process.
A screen under the cutting chamber permits only small resulting particles of solid plastic material to escape the turning reel knives as the plastic material churns within the cutting chamber. The cut solid plastic material which has been reduced to small size drops from the bottom of the cutting chamber through holes in the screen, which are typically about one-quarter (xc2xc) inch diameter and can be from one-eighth (xe2x85x9) inch diameter up to about three-quarters (xc2xe) inch diameter, depending on the size of granules to be produced.
In one of its aspects, this invention provides apparatus for reducing large pieces of solid plastic material to smaller size for recycling. The apparatus includes a longitudinally elongated table having transversely displaced upper and lower segments. The apparatus further includes a cutter mounted for rotation about an axis between the upper and lower table segments and means for rotating the cutter. A movable hopper receives large size plastic pieces to be reduced in size and has an open bottom through which the plastic pieces may contact the table. The apparatus further includes means for supporting the hopper for movement longitudinally between positions over the upper and lower segments of the table.
Preferably, the upper and lower segments of the table are parallel and vertically displaced from one another.
The hopper preferably moves longitudinally over the table and is preferably a generally rectangular box.
The cutter preferably rotates about a transverse axis and preferably has a plurality of blades mounted on a cylindrical member extending transversely between the upper and lower table segments. The blades have cutting edges extending transversely respecting the hopper and spaced from the lower table segment at a position of closest blade approach thereto for passage of reduced size plastic material therebetween.
The apparatus further preferably includes a frame supporting the table.
The apparatus preferably further includes a pair of longitudinally extending tracks connected to the frame for movement of the hopper therealong between positions above the upper and lower table segments and wheels supporting the hopper and riding in the tracks as the hopper moves between the positions above the upper and lower table segments.
Hopper longitudinal movement is preferably powered by an electric motor. The hopper moving means preferably further includes limit switches located at positions defining the limits of hopper longitudinal travel, means for powering movement of the hopper between the respective positions above the upper and lower table segments and means for reversing direction of the hopper movement powering means in response to actuation of the limit switches by the hopper at longitudinal extremities of hopper travel above the table.
Further preferably, the hopper power reversing means further includes means for delaying reversal of the hopper movement powering means for a preselected time upon the hopper contacting one of the limit switches.
The granulator further preferably includes a second granulating assembly having a transversely elongated rotor connected to the shaft of the motor for rotation therewith. A first plurality of cutting knife blades are preferably connected to the rotor at transverse extremities thereof and are preferably oriented with cutting edges of the blades parallel with the motor shaft. A base plate preferably has the motor shaft rotatably journaled therewithin.
A second plurality of cutting knife blades define a circular array connected to the base plate and upstanding therefrom with cutting edges preferably oriented in a direction parallel both with the motor shaft and with the first plurality of cutting knife blade edges. An apertured ring includes notches therein for fitting around and over the second plurality of cutting knife blades.
Annular trunco-cylindrical back plates are supported by the base plate and are positioned radially outboard of the apertured ring and axially substantially aligned with the apertured ring for deflecting granules of plastic material, resulting from cutting action of the first and second pluralities of cutting knife blades, passing through the apertured ring downwardly for collection by a storage bin.
In another of its aspects, this invention provides a method for granulating thermoplastic material for recycling through additional molding by introducing previously coarsely granulated previously molded and/or waste solid thermoplastic material into a cylindrical granulating zone via a cylinder end. The method further encompasses providing at least one stationary knife, preferably having length less than one-quarter (xc2xc) of the diameter the cylindrical granulating zone, at a cylindrical boundary of the granulating zone. The method yet further preferably encompasses moving at least one second knife, preferably having length substantially that of the stationary knife, along the cylindrical boundary of the granulating zone to pass in proximity to the stationary knife and thereby trap portions of the thermoplastic material between the stationary moving knives and cut the portions into granules.
The method further preferably provides an apertured surface as a portion of the cylindrical boundary and further embraces sweeping portions of the material along the apertured surface with a moving cutting knife thereby causing cut portions of the thermoplastic material having granule size less than that of the apertures to pass therethrough for recycling via subsequent molding.